Active transponder, particularly for synthetic aperture radar, or SAR, systems

ABSTRACT

An active transponder for synthetic aperture radar systems includes a receiving antenna for receiving a first radiofrequency signal modulated according to a first train of one or more first pulses, separating means comprising two outputs outputting the first radiofrequency signal, second processing means connected to a first output of the separating means to generate a code synchronized with the first pulses, signal generating means connected to the second output and to the second processing means generate a second radiofrequency signal modulated by the code, and a transmitting antenna means to transmit the second radiofrequency signal to generate, for each one of the first pulses, a sequence of one or more second pulses, the code being synchronized with the second pulses.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage entry on International ApplicationNo, PCT/IT2006/000710 filed Oct. 10, 2006, which claims priority fromItalian patent Application No. RM2005A000282 filed Jun. 3, 2005. Theentire of this application is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an active transponder, in particularusable in synthetic aperture radar systems, or SAR, that allows a SARinterrogator device, carried on an aircraft or satellite, to identify,in a reliable, simple and efficient way, objects associated with thetransponder and to collect information thereon.

BACKGROUND OF THE INVENTION

It is known that the synthetic aperture radar, or SAR, is a microwaveremote sensing active system that, installed onboard aircrafts orsatellites, allows high resolution images of the observed scene to begenerated independently from meteorological conditions and presence ofsunlight. Generally, the wavelength of the microwaves used in a SARsystem ranges from 3 to 30 cm.

Sensing resolution substantially depends on the band of the signaltransmitted by the radar, the antenna beam angle and the distancebetween the radar system and the observed surface.

In applications requiring a continuous monitoring, as for instance inground motion monitoring, the radar is preferably installed onboard asatellite. Notwithstanding the large distance at which such radaroperates, equal to about 800 Km, SAR technology allows a high spatialresolution to be obtained, through an operation of processing the echoreflected by the surface irradiated by the transmitted signal. Inparticular, such processing makes a focusing of the image synthesisingthe reflected echo so as to simulate an antenna aperture significantlylarger than that of the antenna actually installed onboard the aircraft.

One of the characteristics of the SAR systems is their capability tofollow both the amplitude and the phase behaviour of the backscatteredecho. In some applications, such as the Ground Motion Monitoring, thepresence of Permanent Targets, or PTs, over the territory underobservation allows possible ground variations to be detected through theanalysis of the time sequences of the images obtained from the samescene. PTs have radio scattering properties known to the SAR systemwhich remain stable in time.

Present PTs may be passive devices, such as reflectors known to theskilled in the art as “corner reflectors”, or antenna activetransponders. In the latter case, the signal re-transmitted by thetransponder must have the same frequency of the received signal and aphase relationship with the received signal that is stable in time.

German Patent Application No. DE 32 48 879 A1 discloses a systemadopting the technique of identification of the signals transmitted bythe transponder with respect to the neighbouring ones, throughtransmission power and time location.

Such system suffers from some drawbacks. In fact, it subtractsscattering zones, and it does not allow the object to be distinguishedfrom other close ones of the same kind.

U.S. Pat. No. 5,821,895 discloses a system adopting the technique ofidentification of the signals from the transponder through encoding ofthe single pulse. The SAR receiver device carries out correlations alongazimuth and range directions of the received signal with the transmittedand expected SAR signal. These correlations do not detect the presenceof the transponder for low correlation because of the presence of thecode on the signal generated by the transponder. Differently, ifcorrelations take account of the encoding on the transponder signal,then they detect a correlation peak in correspondence with thetransponder position. In this way, the signal transmitted by thetransponder is scarcely visible by the SAR receiver and it has not to bedelayed to minimum scattering instant of poor interest. The system ofU.S. Pat. No. 5,821,895 also adopts the technique of detection of thesingle SAR pulse for applying the code to the signal received by the SARdevice.

Even this system suffers from some drawbacks.

First of all, the adopted technique is subject to detection errors dueto the noise of the transponder receiver. Single pulse detection errorsare terrible because when a pulse is lost, the applied code ismisaligned and correlation is compromised or annulled.

Moreover, the system of U.S. Pat. No. 5,821,895 adopts the technique ofintermediate frequency modulation, hence requiring the use ofsynthesisers and frequency converters which introduce harmonic andspurious components which may jeopardise the signal processing andincrease the background noise.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to allow, in a radarsystem, in particular a SAR system, to identify, in a reliable, simple,and efficient way, even in a high reflection environment, objectsassociated with a transponder and to collect information thereon.

It is specific subject matter of the present invention an activetransponder, particularly for synthetic aperture radar systems, or SAR,comprising:

-   -   receiving antenna means, capable to receive a first        radiofrequency signal modulated according to a first train of        one or more first pulses;    -   first processing means, connected to the receiving means,        capable to amplify and filter the first radiofrequency signal;    -   separating means, connected to the first processing means,        comprising two outputs at each one of which it outputs the        signal coming from the first processing means;    -   second processing means, connected to a first output of the        separating means, capable to generate at least one code        synchronised with the first pulses;    -   signal generating means, connected to a second output of the        separating means and to the second processing means, capable to        generate a second radiofrequency signal modulated by said at        least one code generated by the second processing means;    -   third processing means, connected to the signal generating        means, capable to amplify and filter the second radiofrequency        signal; and    -   transmitting antenna means, capable to transmit the second        radiofrequency signal;        characterised in that the second processing means is capable to        generate, for each one of the first pulses, a sequence of one or        more second pulses, said at least one code being synchronised        with the second pulses.

Always according to the invention, the second processing means maycomprise means for tracking the timing of the first pulses.

Still according to the invention, the second processing means mayfurther comprise:

-   -   envelope detecting means, connected to the first output of the        separating means, capable to generate a sequence of one or more        envelopes for each one of the first pulses;        the means for tracking the timing of the first pulses        comprising:    -   Delay Locked Loop tracking means, connected to the envelope        detecting means, capable to generate a clock signal,        synchronised with said envelopes, and a start signal,        synchronised with the first train of one or more first pulses;    -   code generating means, connected to the Delay Locked Loop        tracking means, capable to generate a not modulated code that        varies in synchronisation with said clock signal according to a        variation rule synchronised with said start signal; and    -   counting means capable to generate a count synchronised with        said clock signal, set at a starting value by said start signal;        the second processing means further comprising:    -   memory means, connected to the counting means, which outputs        data stored at the memory address equal to said count; and    -   first modulating means, connected to the code generating means        and to the memory means, capable to generate said sequence of        one or more second pulses through modulation of said not        modulated code through the data outputted by the memory means.

Furthermore according to the invention, the envelope detecting means maybe capable to generate an envelope for each one of the first pulses.

Always according to the invention, said variation rule may be changeablethrough code changing means.

Still according to the invention, the data stored at the memory address,corresponding to the starting value of said count, may comprise atransponder identifier.

Furthermore according to the invention, the data stored in the memorymeans may comprise data, preferably periodically updated, coming fromone or more sensors associated with the transponder.

Always according to the invention, the Delay Locked Loop tracking meansmay be provided with fast acquisition means, capable to generate saidstart signal.

Still according to the invention, said fast acquisition means maycomprise correlating means, capable to process one or more correlationsbetween the first train of one or more first pulses and one or moresecond trains of one or more third reference pulses.

Furthermore according to the invention, the signal generating means maycomprise modulating means, capable to modulate the signal coming fromthe separating means through said at least one code generated by thesecond processing means, thus obtaining the second radiofrequencysignal.

Always according to the invention, the modulating means may be capableto phase and/or amplitude and/or frequency modulate the signal comingfrom the separating means.

Still according to the invention, the modulating means may be capable toshift the frequency of the signal coming from the separating means.

It is still specific subject matter of the present invention a method ofreceiving a first radiofrequency signal and of transmitting a secondradiofrequency signal, particularly for synthetic aperture radarsystems, or SAR, comprising the following steps:

-   A. receiving a first radiofrequency signal modulated according to a    first train of one or more first pulses;-   B. amplifying and filtering the first radiofrequency signal;-   C. generating at least one code synchronised with the first pulses;-   D. generating a second radiofrequency signal modulated by said at    least one code;-   E. amplifying and filtering the second radiofrequency signal; and-   F. transmitting the second radiofrequency signal;    characterised in that step C comprises:    generating, for each one of the first pulses, a sequence of one or    more second pulses, said at least one code being synchronised with    the second pulses.

Always according to the invention, step C may comprise: tracking atiming of the first pulses.

Still according to the invention, step C may comprise:

-   C.1 generating a sequence of one or more envelopes for each one of    the first pulses;-   C.2 generating a clock signal, synchronised with said envelopes, and    a start signal, synchronised with the first train of one or more    first pulses;-   C.3 generating a not modulated code that varies in synchronisation    with said clock signal according to a variation rule synchronised    with said start signal;-   C.4 generating a count synchronised with said clock signal, set at a    starting value by said start signal;-   C.5 providing stored data corresponding to said count; and-   C.6 generating said sequence of one or more second pulses through    modulation of said not modulated code through the data provided in    sub-step C.5.

Furthermore according to the invention, sub-step C.1 may generate anenvelope for each one of the first pulses.

It is further specific subject matter of the present invention a SARsystem, comprising a transmitting and receiving device and at least oneactive transponder, characterised in that the active transponder is atransponder as previously described.

BRIEF DESCRIPTION OF DRAWINGS

Various exemplary aspects of the systems and methods will be describedin detail, with reference to the following FIGURES, wherein the FIGUREillustrates a schematic block diagram of the active transponderaccording to various aspects of the current invention.

DETAILED DESCRIPTION

The present invention will now be described, by way of illustration andnot by way of limitation, according to its preferred embodiments, byparticularly referring to the sole FIGURE of the enclosed drawings,showing a schematic block diagram of the active transponder according tothe invention.

The inventors have developed a method and a device for locating,identifying, and collecting data on objects, using an active transponderin presence of a transmitting and receiving SAR device carried by anaircraft or a satellite. In particular, the active transponder receivesthe signal transmitted by the transmitting SAR device and provides forre-transmitting it back to the receiving SAR after phase encoding andaddition of a transponder identification code and of local data relatedto the same transponder and to objects, preferably sensors, associatedwith the transponder. A processing device after the receiving SARdevice, capable to decode signals received from transponders, is thusable to extract the identification code and local data.

In particular, the transponder according to the invention adopts anencoding of the SAR pulse different from that of the prior art systems,since it is based on a tracking of pulse timing, instead of thetechnique of detection of the single pulse. Timing tracking greatlyreduces the probability of pulse loss and the consequent correlationloss.

Also, this allows to carry out not only the encoding of the singlepulse, but the encoding inside the same pulse, in particular when theprecision with which signal timing may be reconstructed is sufficientlyhigh. Encoding inside the pulse allows noise immunity and transponderposition measure accuracy to be improved.

Differently from the prior art, the transponder according to theinvention may further use a preamble for the transponder identifyingcode, allowing the transponder to be identified in an area where severaltransponders are present and if the use of the sole code diversity isnot sufficient for characterising the same.

Sensors associated with the transponder may transmit local detectiondata, for instance, environmental information coming from propersensors.

Transmission of identifying code and data occurs through phasemodulation in addition to that produced by the code. Each bit of theidentifying code and the data occupies a number or a fraction of pulsesadequate to ensure an acceptable value of the signal power/noise powerratio, and therefore an acceptable error rate.

In presence of low error rate conditions, transmission of the data andthe identifier does not jeopardise the range and azimuth measure by theSAR receiver device, since identifier and data, once detected, may beused by the SAR receiver device for an intermediate processing. Thisprocessing removes the phase modulation of local data and identifier,known by that time, and it brings the final SAR processing back to theconventional case.

In particular, the transponder according to the invention adopts thetechnique of radio frequency modulation, eliminating the use ofsynthesisers and frequency converters, thus preventing harmonic andspurious components which may jeopardise the signal processing andincrease the background noise from being introduced.

With reference to the FIGURE, it may be observed that the preferredembodiment of the transponder according to the invention comprises areceiving antenna 20 and a transmitting antenna 30, connected to eachother through a cascade of stages shown in detail below. In particular,the receiving antenna 20 and the transmitting antenna 30 are adequatelydecoupled through an arrangement according to different directionsand/or different polarisations.

The receiving antenna 20 is connected to the input of a low noiseamplifier 1, which it provides with the wide band and low power receivedsignal, that is amplified for keeping along the following stages asufficient signal-to-noise ratio or SNR. The output of the low noiseamplifier 1 is connected to the input of a band-pass filter 2, thatoutputs the filtered signal in which components in bands of no interestare excluded.

The output of filter 2 is connected to the input of a two-outputseparator 3, that provides a modulator 4 and an envelope detector 7 withthe amplified and filtered signal, which, in presence of useful signalreceived from a SAR transmitter, comprises a modulated pulse train.

Envelope detector 7 outputs an envelope train corresponding to the pulsetrain received by the 3 to a Delay Locked Loop tracking stage 8.Preferably, detector 7 provides an envelope for each pulse received byseparator 3.

Tracking stage 8 outputs a clock signal and a start signal to a codegenerator 11 and to a counter 12. The clock signal represents the timeaxis of the envelope train generated by envelope detector 7, in terms ofa stabilised pulse sequence synchronised with the same envelopesequence. The start signal marks the time at which the process ofreconstructing the time axis is deemed as reliable, i.e. the startinstant of the SAR signal (that is, of the related pulse train) and themodulating process may be performed.

Code generator 11 outputs a code signal to a code modulator 10 in termsof a pulse train with phase varying at each pulse with variationsynchronised with the pulse train generated by detector 7. The variationrule is internal to code generator 11 and it may be changed through anexternal code change control 25.

Counter 12 outputs the address of the datum to be transmitted to amemory 9, through modulation of the code generated by generator 11.Counter 12 operates on the basis of the clock signal, that causes it toadvance, and of the start signal, that causes it to start.

Memory 9 provides code modulator 10 with the data in the sequencedefined by the address sequence, as received by counter 12. The data areconstituted of a transponder identifier (preferably contained within thefirst address of the sequence) and of a series of data preferably comingfrom sensors (not shown), such as for instance temperature, moisture,power supply, antenna aiming sensors (the data of which areadvantageously periodically updated).

Code modulator 10 outputs a modulated code signal, obtained bymodulating the code signal provided by generator 11 with the data ofmemory 9 retrieved according to the address sequence generated bycounter 12, to signal modulator 4. In other words, the modulated codesignal is obtained starting from the code signal provided by generator11 through further phase variations, applied in the same applicationinstants of the previous ones, produced by the data train alsosynchronised with the pulse train received by the receiving antenna 20.

Signal modulator 4 provides a filter 5 with the SAR signal, i.e. withthe pulse train received by the receiving antenna 20, phase modulatedwith the phase sequence represented by the modulated code signalreceived by modulator 10.

Filter 5 provides an amplifier 6 with the SAR signal that is phasemodulated and restricted to the band of interest, in order to avoidinterferences on other bands and to exploit as much as possible theamplification chain.

Amplifier 6 provides the transmitting antenna 30 with the SAR signalthat is phase modulated, filtered and amplified up to the desired levelfor its transmission towards the space.

The active transponder of the FIGURE further comprises a power supplystage, not shown.

Other embodiments of the transponder according to the invention providethat Delay Locked Loop tracking stage 8 is provided with a fastacquisition device, in order to minimise the time passing from theappearance of the first pulses and the activation of the start signal.The fast acquisition device may be constituted, for instance, of acorrelator detecting the maximum correlation between the received pulsetrain and the expected pulse train. A plurality of correlators may beused in parallel with several timing in order to speed up theacquisition.

Furthermore, signal modulator 4 may apply modulations alternative oradditional to the phase one, in order to increase the decorrelation ofthe transmitted signal with respect to natural reflectors and/or totransmit more information. By way of example, but not by way oflimitation, modulator 4 may apply the amplitude modulation.

Moreover, signal modulator 4 may apply a proper frequency shift allowingthe signal band to be reversed. In this way, for instance, in the caseof chirp-like modulated pulse, the decorrelation between naturalreflectors and the transponder signal increases in terms of larger ratiobetween useful signal and interfering signal and of lower interferenceon the detection of natural reflectors.

Alternatively, signal modulator 4 may be replaced with a signalgenerating stage capable to generate, for each SAR pulse received by thereceiving antenna 20, a pulse of different length and/or differentmodulation and/or different energy, in order to increase thedecorrelation of the transmitted signal with respect to naturalreflectors and/or to transmit more information. By way of example andnot by way of limitation, such stage could generate a frequencymodulated signal.

The preferred embodiments have been above described and somemodifications of this invention have been suggested, but it should beunderstood that those skilled in the art can make variations andchanges, without so departing from the related scope of protection, asdefined by the following claims.

1. Active transponder comprising: receiving antenna means, configured toreceive a first radiofrequency signal modulated according to a firsttrain of one or more first pulses; first processing means, connected tothe receiving means, configured to amplify and filter the firstradiofrequency signal; separating means, connected to the firstprocessing means, comprising two outputs each one of which outputs thesignal coming from the first processing means; second processing means,connected to a first output of the separating means, configured togenerate at least one code synchronised with the first pulses; signalgenerating means, connected to a second output of the separating meansand to the second processing means, configured to generate a secondradiofrequency signal modulated by said at least one code generated bythe second processing means and having at least one of a differentfrequency value from a frequency value of the first radiofrequency and adifferent phase value from a phase value of the first radiofrequency;third processing means, connected to the signal generating means,configured to amplify and filter the second radiofrequency signal; andtransmitting antenna means, configured to transmit the secondradiofrequency signal; wherein the second processing means is configuredto generate, for each one of the first pulses, a sequence of one or moresecond pulses, said at least one code being synchronised with the secondpulses, the transponder being characterised in that the secondprocessing means further comprises: envelope detecting means, connectedto the first output of the separating means, configured to generate asequence of one or more envelopes for each one of the first pulses;means for tracking the timing of the first pulses comprising: DelayLocked Loop tracking means, connected to the envelope detecting means,configured to generate a clock signal, synchronised with said envelopes,and a start signal, synchronized with the first train of one or morefirst pulses; code generating means, connected to the Delay Locked Looptracking means, configured to generate a not modulated code that variesin synchronisation with said clock signal according to a variation rulesynchronised with said start signal; and counting means configured togenerate a count synchronised with said clock signal, set at a startingvalue by said start signal; the second processing means furthercomprising: memory means, connected to the counting means, which outputsdata stored at the memory address equal to said count; and firstmodulating means, connected to the code generating means and to thememory means, configured to generate said sequence of one or more secondpulses through modulation of said not modulated code through the dataoutputted by the memory means.
 2. Transponder according to claim 1,characterised in that the envelope detecting means are configured togenerate an envelope for each one of the first pulses.
 3. Transponderaccording to claim 1, characterised in that said variation rule ischangeable through code changing means.
 4. Transponder according toclaim 1, characterised in that the data stored at the memory address,corresponding to the starting value of said count, comprise atransponder identifier.
 5. Transponder according to claim 1,characterised in that the data stored in the memory means comprise data,periodically updated, coming from one or more sensors associated withthe transponder.
 6. Transponder according to claim 1, characterised inthat the Delay Locked Loop tracking means is provided with fastacquisition means, configured to generate said start signal. 7.Transponder according to claim 6, characterised in that said fastacquisition means comprises correlating means, configured to process oneor more correlations between the first train of one or more first pulsesand one or more second trains of one or more third reference pulses. 8.Transponder according to claim 1, characterised in that the signalgenerating means comprises modulating means, configured to modulate thesignal coming from the separating means through said at least one codegenerated by the second processing means, thus obtaining the secondradiofrequency signal.
 9. Transponder according to claim 8,characterised in that the modulating means is configured to phase and/oramplitude and/or frequency modulate the signal coming from theseparating means.
 10. Transponder according to claim 8, characterised inthat the modulating means is configured to shift the frequency of thesignal coming from the separating means.
 11. A synthetic aperture radarsystem, comprising a transmitting and receiving device and at least oneactive transponder, characterised in that the active transponder is atransponder according to claim
 1. 12. Method of receiving a firstradiofrequency signal and of transmitting a second radiofrequencysignal, for synthetic aperture radar systems, or SAR, comprising thefollowing steps: A. receiving a first radiofrequency signal modulatedaccording to a first train of one or more first pulses; B. amplifyingand filtering the first radiofrequency signal; C. tracking a timing ofthe first pulses and generating at least one code synchronised with thefirst pulses; D. generating a second radiofrequency signal modulated bysaid at least one code that has at least one of a different frequencyvalue from a frequency value of the first radiofrequency and a differentphase value from a phase value of the first radiofrequency; E.amplifying and filtering the second radiofrequency signal; and F.transmitting the second radiofrequency signal; wherein step C comprises:generating, for each one of the first pulses, a sequence of one or moresecond pulses, said at least one code being synchronised with the secondpulses, the method being characterised in that step C comprises: C.1generating a sequence of one or more envelopes for each one of the firstpulses; C.2 generating a clock signal, synchronised with said envelopes,and a start signal, synchronised with the first train of one or morefirst pulses; C.3 generating a not modulated code that varies insynchronisation with said clock signal according to a variation rulesynchronised with said start signal; C.4 generating a count synchronisedwith said clock signal, set at a starting value by said start signal;C.5 providing stored data corresponding to said count; and C.6generating said sequence of one or more second pulses through modulationof said not modulated code through the data provided in sub-step C.5.13. Method according to claim 12, characterised in that sub-step C.1generates an envelope for each one of the first pulses.